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PRINCIPLES
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"In truth, I found myself incorrigible with respect to *Order*; and
now I am grown old and my memory bad, I feel very sensibly the want of
it. But, on the whole, though I never arrived at the perfection I had
been so ambitious of obtaining, but fell far short of it, yet I was,
by the endeavour, a better and happier man than I otherwise should
have been if I had not attempted it; as those who aim at perfect
writing by imitating the engraved copies, though they never reach the
wished-for excellence of those copies, their hand is mended by the
endeavor, and is tolerable while it continues fair and legible."
-- Benjamin Franklin in his autobiography
"'Signs make humans do things,' said Nisodemus, 'or stop doing things.
So get to work, good Dorcas. Signs. Um. Signs that say *No*.'"
-- Terry Pratchett, _Diggers_
There are some principles which I'd like to see used in the
maintenance of SBCL:
1. conforming to the standard
2. being maintainable
a. removing stale code
b. When practical, important properties should be made manifest in
the code. (Putting them in the comments is a distant second best.)
i. Perhaps most importantly, things being the same (in the strong
sense that if you cut X, Y should bleed) should be manifest in
the code. Having code in more than one place to do the same
thing is bad. Having a bunch of manifest constants with hidden
relationships to each other is inexcusable. (Some current
heinous offenders against this principle are the memoizing
caches for various functions, and the LONG-FLOAT code.)
ii. Enforcing nontrivial invariants, e.g. by declaring the
types of variables, or by making assertions, can be very
helpful.
c. using clearer internal representations
i. clearer names
A. more-up-to-date names, e.g. PACKAGE-DESIGNATOR instead
of PACKAGELIKE (in order to match terminology used in ANSI spec)
B. more-informative names, e.g. SAVE-LISP-AND-DIE instead
of SAVE-LISP or WRAPPER-INVALID rather than WRAPPER-STATE
C. families of names which correctly suggest parallelism,
e.g. CONS-TO-CORE instead of ALLOCATE-CONS, in order to
suggest the parallelism with other FOO-TO-CORE functions
ii. clearer encodings, e.g. it's confusing that WRAPPER-STATE in PCL
returns T for valid and any other value for invalid; could
be clarified by changing to WRAPPER-INVALID returning a
generalized boolean; or e.g. it's confusing to encode things
as symbols and then use STRING= SYMBOL-NAME instead of EQ
to compare them.
iii. clearer implementations, e.g. cached functions being
done with HASH-TABLE instead of hand-coded caches
d. informative comments and other documentation
i. documenting things like the purposes and required properties
of functions, objects, *FEATURES* options, memory layouts, etc.
ii. not using terms like "new" without reference to when.
(A smart source code control system which would let you
find when the comment was written would help here, but
there's no reason to write comments that require a smart
source code control system to understand..)
e. using functions instead of macros where appropriate
f. maximizing the amount of stuff that's (broadly speaking) "table
driven". I find this particularly helpful when the table describes
the final shape of the result (e.g. the package-data-list.lisp-expr
file), replacing a recipe for constructing the result (e.g. various
in-the-flow-of-control package-manipulation forms) in which the
final shape of the result is only implicit. But it can also be very
helpful any time the table language can be just expressive enough
for the problem at hand.
g. using functional operators instead of side-effecting operators
where practical
h. making it easy to find things in the code
i. defining things using constructs which can be understood by etags
i. using the standard library where possible
i. instead of hand-coding stuff
(My package-data-list.lisp-expr stuff may be a bad example as of
19991208, since the system has evolved to the point where it
might be possible to replace my hand-coded machinery with some
calls to DEFPACKAGE.)
j. more-ambitious dreams..
i. fixing the build process so that the system can be bootstrapped
from scratch, so that the source code alone, and not bits and
pieces inherited from the previous executable, determine the
properties of the new executable
ii. making package dependencies be a DAG instead of a mess, so
the system could be understood (and rebuilt) in pieces
iii. moving enough of the system into C code that the Common Lisp
LOAD operator (and all the symbol table and FOP and other
machinery that it depends on) is implemented entirely in C, so
that GENESIS would become unnecessary (because all files could
now be warm loaded)
3. being portable
a. In this vale of tears, some tweaking may be unavoidably required
when making software run on more than one machine. But we should
try to minimize it, not embrace it. And to the extent that it's
unavoidable, where possible it should be handled by making an
abstract value or operation which is used on all systems, then
making separate implementations of those values and operations
for the various systems. (This is very analogous to object-oriented
programming, and is good for the same reasons that method dispatch
is better than a bunch of CASE statements.)
4. making a better programming environment
a. Declarations *are* assertions! (For function return values, too!)
b. Making the debugger, the profiler, and TRACE work better.
c. Making extensions more comprehensible.
i. Making a smaller set of core extensions. IMHO the high level
ones like ONCE-ONLY and LETF belong in a portable library
somewhere, not in the core system.
ii. Making more-orthogonal extensions. (e.g. removing the
PURIFY option from SAVE-LISP-AND-DIE, on the theory that
you can always call PURIFY yourself if you like)
iii. If an extension must be complicated, if possible make the
complexity conform to some existing standard. (E.g. if SBCL
supplied a command-line argument parsing facility, I'd want
it to be as much like existing command-line parsing utilities
as possible.)
5. other nice things
a. improving compiled code
i. faster CLOS
ii. bigger heap
iii. better compiler optimizations
iv. DYNAMIC-EXTENT
b. increasing the performance of the system
i. better GC
ii. improved ability to compile prototype programs fast, even
at the expense of performance of the compiled program
c. improving safety
i. more graceful handling of stack overflow and memory exhaustion
ii. improving interrupt safety by e.g. locking symbol tables
d. decreasing the size of the SBCL executable
e. not breaking old extensions which are likely to make it into the
new ANSI standard
6. other maybe not-so-nice things
a. adding whizzy new features which make it harder to maintain core
code. (Support for the debugger is important enough that I'll
cheerfully make an exception. Multithreading might also be
sufficiently important that it's probably worth making an exception.)
The one other class of extensions that I am particularly interested
is CORBA or other standard interface support, so that programs can
more easily break out of the Lisp/GC box to do things like graphics.
("So why did you drop all the socket support, Bill?" I hear you
ask. Fundamentally, because I have 'way too much to maintain
already; but also because I think it's too low-level to add much
value. People who are prepared to work at that level of abstraction
and non-portability could just code their own wrapper layer
in C and talk to it through the ALIEN stuff.)
7. judgment calls
a. Sharp, rigid tools are safer than dull or floppy tools. I'm
inclined to avoid complicated defaulting behavior (e.g. trying
to decide what file to LOAD when extension is not specified) or
continuable errors, preferring functions which have simple behavior
with no surprises (even surprises which are arguably pleasant).
CMU CL maintenance has been conservative in ways that I would prefer to
be flexible, and flexible in ways that I'd prefer to be conservative.
CMU CL maintainers have been conservative about keeping old code and
maintaining the old structure, and flexible about allowing a bunch of
additional stuff to be tacked onto the old structure.
There are some good things about the way that CMU CL has been
maintained that I nonetheless propose to jettison. In particular,
binary compatibility between releases. This is a very handy feature,
but it's a pain to maintain. At least for a while, I intend to just
require that programs be recompiled any time they're to be used with a
new version of the system. After a while things might settle down to
where recompiles will only be required for new major releases, so
either all 3.3.x fasl files will work with any 3.3.y runtime, or all
3.w.x fasl files will work with any 3.y.z runtime. But before trying
to achieve that kind of stability, I think it's more important to
be able to clean up things about the internal structure of the system.
Aiming for that kind of stability would impair our ability to make
changes like
* cleaning up DEFUN and DEFMACRO to use EVAL-WHEN instead of IR1 magic;
* reducing the separation between PCL classes and COMMON-LISP classes;
* fixing bad FOPs (e.g. the CMU CL fops which interact with the *PACKAGE*
variable)